1. Field of the Invention
This invention relates to laser systems in which the output bandwidth is restricted by feeding laser radiation back to the laser source via an optical diffraction grating, and more particularly to the use of this technique for high power laser diode arrays.
2. Description of the Related Art
Diffraction gratings have been used in the past in external cavity laser systems to significantly narrow the output bandwidth from a diode laser, particularly a multiple-stripe quantum well heterostructure diode laser. By reflecting back to the laser a small fraction of its output power, the line width can be significantly reduced. A review of the techniques that have been used to narrow the spectra of diode lasers is provided in Wieman et al., "Using Diode Lasers for Atomic Physics", Rev. Sci. Instrum., Vol. 62, No. 1, January 1991, pages 1-20. The developments covered in this review are restricted to single element, single mode devices that produce very narrow spectral outputs at low power levels.
Applications of the diffraction grating feedback technique to diode lasers have been limited in general to narrow-stripe lasers, with gain channel widths of about 3-6 microns (see Patrick et al., "Frequency Stabilization of a Diode Laser Using Simultaneous Optical Feedback From a Diffraction Grating and a Narrow Band Fabry-Perot Cavity", Rev. Sci. Instrum., Vol. 62, No. 11, November 1991, pages 2593-2595), or at most to multiple stripe, optically coupled, broad-area lasers with gain channel widths of about 100 microns (see Epler et al., "Super Modes of Multiple-Stripe Quantum-Well Heterostructure Laser Diodes Operated (CW, 300K) in an External-Grating Cavity", Journal of Applied Physics, Vol. 57, No. 5, Mar. 1, 1985, pages 1489-1494).
Work in the area has centered upon achieving an extremely narrow emission with a single longitudinal mode, or upon examining detailed mode structure in a research context. Output power has not been a primary concern, and the devices have been limited to tens of mW. For example, in Harvey et al., "External-Cavity Diode Laser Using a Grazing-Incidence Diffraction Grating", Optics Letters, Vol. 16, No. 12, Jun. 15, 1991, pages 910-912, a diffraction grating was used to reduce the line width of a GaAlAs diode laser by a factor of more than 1,000 from 40 MHz to less than 10 kHz, while the output power was limited to less than 20 mW; in the Epler article mentioned above a bandwidth narrowing from 5-mode 12 Angstroms to single-mode 0.2 Angstroms was achieved with a diode power output of 170 mW.
Even if output power were a primary goal, single element, broad-area lasers are ultimately limited to powers of about 1 to 2 watts by thermal dissipation. However, there are applications which require significantly greater amounts of power. In particular, optically pumped upconversion lasers require high power, spectrally narrow diode pumps to achieve the high output powers and good electrical-to-optical conversion efficiencies that are necessary for full commercial value. Upconversion lasers are used to convert infrared to visible radiation; a full color upconversion laser that is pumped by a single wavelength infrared laser is described for example in McFarlane, U.S. Pat. No. 5,008,890, assigned to Hughes Aircraft Company, the assignee of the present invention. Arrays of spatially separated (and thus optically isolated) lasers which extend over distances on the order of 1 cm are required to produce beam powers in the 5-20 Watt range that is desirable for pumping an upconversion laser. A diode laser has previously been used to pump an upconversion laser, but only in the context of a single 0.1 Watt narrow-stripe, single mode diode pump laser that achieved an output power of only 2 mW at 551 nm (Hebert et al., "Diode-Laser-Pumped 551 nm Upconversion Laser in YLiF.sub.4 :Er.sup.3+ ", Proceedings of Advanced Solid State Laser Conference Six, Optical Society of America, Washington, D.C., 1990, pages 379-383).
The techniques that have previously been used to narrow the emission spectra of single diode lasers are not directly applicable to a much higher power laser array. This is illustrated in FIGS. 1-3, in which prior single-diode narrow bandwidth systems are shown in FIGS. 1 and 2, with FIG. 3 illustrating the consequences of using the same systems in a laser array environment. In FIG. 1 a laser diode 2 is shown emitting a diverging output light beam 4 that is collimated by a spherical lens 6 (the term "light" herein refers to optical emissions in general, and is not limited to visible light). The collimated beam, which is emitted from the laser 2 in a polarized state, is transmitted through a half wave plate 8 to a polarizing beam splitter 10 that divides the beam into an output component 12 that is transmitted through the beam splitter, and a feedback element 14 that is reflected by the beam splitter to a diffraction grating 16. The angular orientation of half wave plate 8 determines the ratio between the output and feedback beam components 12 and 14, respectively. The feedback component 14, which normally represents a minority of the beam power, is retro-reflected from the diffraction grating 16 in the usual manner, returned to polarizing beam splitter 10, and reflected therefrom back through half wave plate 8 and lens 6, which focuses it into the laser 2. This feedback of a portion of the laser output has been found to produce a distinct narrowing of the laser's emission spectrum. It should be noted that the laser beam is symmetrically centered upon a system axis 18, such that the feedback beam component 14 is accurately redirected along a return path back to the laser.
FIG. 2 shows another prior embodiment in which the output of laser 2 is again collimated by lens 6. In this version, however, a diffraction grating 16' is placed directly in the path of the beam that emanates from lens 6. The diffraction grating 16' is partially reflective and partially transmissive, such that the system output is the beam component 12' that is transmitted through the grating; the feedback component 14' is the portion that is reflected back along the system axis 18', through the lens 6 and into the laser 2. The division of the original laser beam into the output and feedback components is controlled by an appropriate selection of the depths and shapes of the grooves (also called rulings) 20 in the diffraction grating 16', in a known fashion. As in FIG. 1, the beam symmetry about the system axis 18' assures a return of the feedback component to the source laser.
The situation changes if a multiple laser array is substituted for the single laser 2 for the purpose of achieving higher beam powers. This situation is illustrated in FIG. 3 for a system that corresponds to that shown in FIG. 2; a similar response is produced if a multiple Laser array is substituted into the system of FIG. 1. The laser array 22 is shown as a lateral array of spatially separated lasers 22a, 22b and 22c that are fabricated in a conventional manner and optically isolated from each other. Although only three lasers are shown, a greater number would normally be required to achieve the high powers necessary for satisfactory upconversion laser pumping. The lasers in FIG. 3 are viewed from above, with the plane of the lasers parallel to the laser plane. For purposes of further discussion, the term "vertical" refers to a direction perpendicular to the plane of the laser diode junctions, while the term "horizontal" refers to a direction parallel to the laser plane.
A system axis 18" is shown extending from the central laser 22b through the center of a spherical lens 6' (shown larger than lens 6 in FIGS. 1 and 2 to accommodate the dimension of the laser array 22) and on to the diffraction grating 16' with grooves 20. Considering the uppermost laser 22a, its output beam 4' is approximately collimated by lens 6'. However, since the laser 22a is off the lens axis, the beam is directed by the lens at an angle to its axis, rather than parallel as in FIGS. 1 and 2. The redirected beam 24 strikes the diffraction grating 16' at a non-perpendicular angle to the diffraction grooves 20. The horizontal component of light which strikes the diffraction grating at this angle is reflected off the grating at an equal angle on the opposite side of the system axis 18", resulting in a reflected beam 26 that does not retrace the original beam path and in fact can miss the lens 6' entirely. The beam's transmitted component 28 leaves the system at an angle to the system axis, but this can be handled with a corresponding rearrangement of the elements external to the system. However, the lack of retro-reflection for the reflected beam component 26 prevents that component from returning to its source laser. As a result the desired narrowing of the output bandwidth is not achieved.